The innovative Metris Cross Scanner is used at Volvo Cars Gent to further accelerate the design-through-manufacturing process for its brand new Volvo XC60 crossover vehicle. By digitizing physical sheet metal and plastic body parts and virtually assembling vehicle bodies in software, Volvo engineers completed pre-production geometry verification nearly twice as fast! 3D laser scanning technology, point cloud processing and virtual assembly shortened physical evaluation of prototypes and eliminates the need for costly specialized verification tooling.

Body assembly on the critical vehicle development path

Volvo Cars built up a solid reputation in terms of vehicle safety, environment and design. Recently, the Swedish car maker released the brand new Volvo XC60, which currently stirs up the new crossover vehicle segment. The vehicle body of this innovative car required the coordination of many different groups to design and manufacture. Sheet metal stamping and welding in combination with the use of new materials and joining technologies set ever-tougher geometric challenges. Process and product tolerances as well as material and equipment behavior can influence body geometry, when shifting from vehicle body CAD model to physical nominal model and finally to serial-produced car. The position of edges, holes and other geometric features plays an essential role in correctly assembling the different body parts of a passenger vehicle.

In 2005, Belgium-based Volvo Cars Gent and Metris (Leuven) participated in a project that aimed to streamline the pre-production phases by simplifying the geometric body verification process. Both companies joined forces to develop a new geometric verification method for vehicle bodies, which builds on a digital inspection process using 3D scanning and virtual assembly. This method provides better insight and effectiveness compared to traditional body tuning, which involves extensive tactile inspection, physical part conflict analysis and complex verification tooling.

Ground-breaking 3D cross laser scanner technology

In close collaboration with Volvo Cars, Metris optimized its existing cross scanner to match the performance level required to drive the new geometric verification method. “We jointly integrated the laser scanner for use on horizontal-arm CMMs, and increased the scanner’s field-of-view depth. The increased scanning standoff distance range offers higher measuring flexibility and better access to clamped body components. The cross scanner incorporates 3 laser beam / digital camera sets, each shifted 120 degrees in position. This allows the laser scanner to capture slots, sleeves, holes and other features in a single scan. Although inherently designed for scanning geometric features, the cross scanner is also suitable for digitizing 3D surfaces and edges. The positions of features and edges are imperative to correctly mate parts and assemble car bodies.

The development work also impacted laser optics technology and digital data processing. The cross scanner design has been enhanced to flexibly deal with all material types and colors without the use of spray. Now, reflective sheet metal as well as painted surfaces can be captured quickly and reliably. Laser scanning generates point cloud data at high scan rates, which by far outperform tactile point-by-point acquisition technology. Furthermore, it is much easier to define the linear and polygon scanner travel paths than to specify individual touch sensor points for a tactile inspection job.

Faster and better Volvo XC60 body geometry verification

In the pre-production stage at Volvo Cars, metrology engineers scan sheet metal and castings (steel and aluminum) as well as composite and plastic body parts. After acquiring data at approximately 20 micron accuracy, they filter the resulting point cloud, and analyze geometry against nominal CAD data. Volvo Cars relies on digital graphic reports to evaluate the parts, and streamline supplier interaction with regards to adjusting molding and stamping equipment. Digital component verification only requires standard holding fixtures, whereas traditional inspection methods demand costly dedicated positioning and fixation tooling.

After digitizing individual parts, engineers align and virtually assemble sheet metal, interior, exterior and chassis components in software in order to build a complete virtual vehicle body. Even before body parts are physically assembled, the new geometric verification approach gives already information about potential part fitting issues. To run specialized investigations, virtual body assembly models are loaded into dedicated software for reverse engineering, variation analysis, and spring-back prediction, for example. Analysis between scanned and numerical vehicle body models enables us to efficiently tune component geometry to fall within the assembly processing window.

Verifying surfaces and features using handheld laser scanners

The collaboration project with Volvo Cars also contributed to the development of K-Scan, a handheld laser scanner with a single laser stripe for in-situ inspection. An optical CMM continuously tracks the scanner so that the operator can freely walk around and take scans in an area that spans an entire vehicle. Volvo engineers use K-Scan to verify flush & gap, body deformation and static/dynamic geometry on prototype or early production vehicles. Color-coded visual inspection reports illustrate how flush & gap evolves along complete spines in between hood and front fender, for example. Optical handheld verification also includes special cases where manual methods fall short, such as zero gaps, or in case an urgent issue comes up that needs fast troubleshooting.

In summary, the new process reduces times for matching loops, and realizes an important cost reduction for test materials and screwed body, nominal blue bucks and dedicated fixtures. Virtual verification on the basis of 3D scanning is a major step forward. This project received the Henry Ford Technology Award in Detroit. 3D scanning covers reverse engineering of clay models, virtual geometry verification of vehicle body structures, and geometric feedback data collection on finished cars.

Reduction in number of geometry iteration loops

When preparing production rollout for the Volvo C30 in 2006, the virtual body geometry method has been applied and evaluated for the first time. In parallel, traditional tactile verification methods were performed to set benchmarks in terms of inspection precision and throughput. When ramping up Volvo XC60 production in 2008, it was actually managed to reduce geometry iteration loops and the lead-time of individual loops Fewer physical evaluation prototypes also reduce material scrap and decreases expenditure of complex verification tooling, such as body-in-white cubing.

3D scanning technologies are already well accepted at Volvo Cars where they are used in different stages of the car manufacturing process. Non-contact metrology is systematically applied in the early design stages when engineering styling by digitizing clay models. In pre-production, engineering intensively digitizes body parts and body-in-white structures to optimize part manufacturing and assembly. After kicking off serial production, specific aspects of car components or full cars are scanned to serve as SPC samples for quality monitoring and product audit purposes. For the future, an important role for laser scanning is seen as a key enabler of in-line quality control.